Chapter 37 / The Cerebellum 921
Figure 37–10 Synaptic organization of the cerebellar micro-
circuit.Excitation and inhibition converge both in the cerebellar
cortex and in the deep nuclei. Recurrent loops involve Golgi
cells within the cerebellar cortex and the inferior olive outside
the cerebellum. (Adapted, with permission, from Raymond,
Lisberger, and Mauk 1996. Copyright © 1996 AAAS.)
浦肯野细胞
攀缘
纤维
下
橄榄核
细胞
小脑
深部核团
苔藓
纤维
颗粒
细胞
篮状细胞/
星状细胞
平行纤维
高尔基
细胞
–
–
–
–
+
+
+
Parallel Feedforward Excitatory and Inhibitory Pathways
The excitatory inputs relayed from mossy fibers to
granule cells to Purkinje cells work in parallel with
feedforward inhibitory inputs through the two molec-
ular layer interneurons, the stellate and basket cells.
Both of these interneurons receive inputs from parallel
fibers and inhibit Purkinje cells, but they have quite
different architectures.
The short axons of stellate cells contact the nearby
dendrites of Purkinje cells. Thus, a stellate cell acts
locally in the sense that it and the Purkinje cell it inhib-
its are excited by the same parallel fibers. In contrast,
a basket cell acts more widely. Its axon runs perpen-
dicular to the parallel fibers (Figure 37–8) and creates
flanks of inhibition on Purkinje cells that receive input
from parallel fibers other than those that excite the bas-
ket cell. Stellate cells affect Purkinje cells via synapses
that are on distal dendrites, whereas basket cells make
powerful synapses on the cell body of Purkinje cells
and seem to be positioned for a powerful influence on
Purkinje cell simple spiking. Remarkably, even 60 years
after the architecture of the cerebellar microcircuit
was described, the functional role of molecular layer
interneurons remains a mystery.
Convergence of excitatory and inhibitory path-
ways is a predominant feature also in the deep cer-
ebellar nuclei. Here, inhibitory inputs from Purkinje
cells converge with excitatory inputs from axon col-
laterals of mossy and climbing fibers (Figure 37–10).
Thus, a mossy fiber affects target neurons in the deep
nuclei in two ways: directly by excitatory synapses and
indirectly by pathways through the cerebellar cortex
and the inhibitory Purkinje cells. Neurons of the deep
cerebellar nuclei are active spontaneously even in the
absence of synaptic inputs, so the inhibitory output of
the Purkinje cells both modulates this intrinsic activ-
ity and sculpts the excitatory signals transmitted from
mossy fibers to the deep nuclei. In almost all parts of
the cerebellum, collaterals from climbing fibers to the
deep cerebellar nuclei create the opportunity for a sim-
ilar interaction of excitatory and inhibitory inputs.
Recurrent Loops
An important recurrent loop is contained entirely
within the cerebellar cortex and employs Golgi cells to
sculpt the activity of the granule cells, the input ele-
ments in the cerebellar cortex. Golgi cells receive a few
large excitatory inputs from mossy fibers, many smaller
excitatory inputs from parallel fibers, and inhibitory
inputs from neighboring Golgi cells. The GABAergic
terminals from Golgi cells inhibit granule cells (Figure
37–10) and thereby regulate the activity of granule cells
and the signals conveyed by the parallel fibers. This
loop is evidence that important processing may occur
within the granular layer. It may shorten the duration
of bursts in granule cells, limiting the magnitude of the
excitatory response of granule cells to their mossy fiber
inputs, or could ensure that the granule cells respond
only when a certain number of their mossy fiber inputs
are active.
A second recurrent loop provides Purkinje cells
with a way to regulate their own climbing fiber inputs
(Figure 37–10). Purkinje cells inhibit GABAergic inhib-
itory neurons in the deep cerebellar nuclei that project
to the inferior olive. When the simple-spike firing of a
group of Purkinje cells decreases, the activity of these
inhibitory interneurons increases, leading to decreases
in the excitability of neurons in the inferior olive. The
decreased excitability of the inferior olive reduces both
the probability of action potentials in climbing fib-
ers that project to the original group of Purkinje cells
and the duration of each burst of climbing fiber action
Kandel-Ch37_0908-0931.indd 921 11/12/20 12:18 PM